Photovoltaic power plants with light concentration reduce usage of expensive semiconductor resources. The typical approach may comprise a plurality of “discrete” cells with enhanced efficiency arranged on an appropriate substrate in closely packed arrays, each with an active area that may be from about 0.5 to about 4 cm2, or even larger.
The cells may be of various types, for example, a stack structure of high efficiency double or triple junction cells comprising a topmost semiconductor layer of Gallium Indium Diphosphide (GaInP2) over an intermediate semiconductor layer of Gallium Arsenide (GaAs), which is grown over a substrate of intermediate monocrystalline Germanium (Ge). The junctions, being between the top and the intermediate semiconductor layer and between the intermediate semiconductor layer and the substrate layer, may comprise two tunneling junctions. A rear metal layer and a metal contact formed on the top semiconductor layer may comprise the electric terminals of the cells. The surface area of the top semiconductor layer of Gallium Indium Diphosphide, not being covered by the metal contact, represents the active area of the cell and may be provided with a reflective transparent film.
Being possible to selectively optimize the structure of single cells to enhance their conversion efficiency for a specific region of the solar radiation spectrum, the overall conversion efficiency of the system may be significantly increased by splitting the spectrum of the collected radiation and directing beams of different parts of the spectrum toward the active surfaces of respectively dedicated cells to exploit the greatest sensitivity for radiation with wavelengths in the specific spectrum of maximum efficiency of the different cells.
To this end, parabolic mirrors for collecting and concentrating radiation in a beam have been disclosed. The beam is suitable for illuminating the active areas of the cells. The mirror surface may comprise a plurality of planar polyhedral mirrors that approximate a parabolic surface, and each planar polyhedral mirror tile forming the parabolic surface has superposed semi-reflective dichroic filter layers over a reflective substrate layer. In this way, the three or more reflective surfaces of different focal distances concentrate the solar spectrum in three distinct portions. The relative beams of which are directed onto three cells or arrays of cells with characteristics optimized for the specific region of the spectrum.